Programmable thermostat

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A touch-screen programmable thermostat in programming mode.

A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats may also be called setback thermostats or clock thermostats.

1 Benefits
2 Construction and features
3 External links

[edit] Benefits

Heating and cooling losses from a building (or any other container) become greater as the difference in temperature increases. A programmable thermostat allows reduction of these losses by allowing the temperature difference to be reduced at times when the reduced amount of heating or cooling would not be objectionable.

For example, during the air conditioning season, a programmable thermostat used in a home may be set to allow the temperature in the house to rise during the workday when no one will be at home. It may then be set to turn on the air conditioning before the arrival of occupants, allowing the house to be cool upon the arrival of the occupants while still having saved air conditioning energy during the peak outdoor temperatures. The reduced cooling required during the day also decreases the demands placed upon the electrical supply grid.

Conversely, during the heating season, the programmable thermostat may be set to allow the temperature in the house to drop when the house is unoccupied during the day and also at night after all occupants have gone to bed, re-heating the house prior to the occupants arriving home in the evening or waking up in the morning. Since most people sleep better when a room is cooler and the temperature differential between the interior and exterior of a building will be greatest on a cold winter night, this reduces energy losses.

Similar scenarios are available in commercial buildings, with due consideration of the building's occupancy patterns.

With the difference in temperature between interior and exterior being reduced, it is not unusual to achieve 30% energy usage reductions with the default program on residential thermostats, though adjusting the default settings may improve or reduce energy usage.

[edit] Construction and features

[edit] Clock thermostats

The most basic clock thermostats may only implement one program with two periods (a hotter period and a colder period), and the same program is run day after day. More sophisticated clock thermostats may allow four or more hot and cold periods to be set per day. Usually, only two distinct temperatures (a hotter temperature and a colder temperature) can be set, even if multiple periods are permitted. The hotter and colder temperatures are usually established simply by sliding two levers along an analogue temperature scale, much the same as in a conventional (non-clock) thermostat.

This design, while simple to manufacture and relatively easy to program, sacrifices comfort on weekends since the program is repeated each of the seven days of the week with no variation. To overcome this deficit, a push-button is sometimes provided to allow the user to explicitly switch (once) the current period from hot period to a cold period or vice-versa; the usual use of this button is to over-ride a "set back" that takes place during the workday when the home is normally unoccupied .

The clock mechanism is electrical, and two methods have commonly been used to drive it:

A permanent source of 24 volts AC is provided to the thermostat, or

A rechargeable battery in the thermostat operates the clock. The battery charges when the thermostat is not calling for heat (and 24 VAC is available across the thermostat's terminals) and discharges to operate the clock when the thermostat is "calling" for heating or cooling.

[edit] Digital thermostats

Digital thermostats may implement the same functions, but most provide more versatility. For example, they commonly allow setting temperatures for four periods each day, and rather than being limited to a single "hotter" temperature and a single "colder" temperature, digital thermostats usually allow each period to be set to a unique temperature. The periods are commonly labeled "Morning", "Day", "Evening", and "Night", although nothing constrains the time intervals involved. Digital thermostats usually allow the user to over-ride the programmed temperature for the period, automatically resuming programmed temperatures when the next period begins. A function to "hold" (lock-in) the current temperature is usually provided as well; in this case, the over-ride temperature is maintained until the user cancels the hold.

As with clock thermostats, basic digital thermostats may have just one cycle that is run every day of the week. More-sophisticated thermostats may have a weekday schedule and a separate weekend schedule (so-called "5-2" setting) or separate Saturday and Sunday schedules (so-called "5-1-1" settings), while other thermostats will offer a separate schedule for each day of the week ("7 day" settings). Often, a manufacturer will sell three similar thermostats offering each of those levels of sophistication and there is no physical difference in the thermostats other than the factory programming and the price.

Most digital thermostats have entirely separate programs for heating and cooling.

Digital thermostats may also have an air filter change reminder; this counts the accumulated run-time of the heating/cooling system and reminds the user every (say) thousand hours that it is now time to change the filter. The feature often displays the accumulated run-time.

Digital thermostats are usually powered either of three ways:

A sophisticated power circuit operates from the 24 VAC supply when the thermostat is not calling, and operates from the current flowing in the thermostat circuit when the thermostat is calling. A battery is used to provide back-up during power failures.

A rechargeable battery operates the thermostat just as in the clock thermostat, charging when the thermostat is not calling and discharging while the thermostat is calling.

A non-rechargeable battery always powers the thermostat. To limit the amount of power drawn from the battery, such thermostats use an impulse relay that does not require the continuous application of power to the relay's coil. These thermostats can be used on millivolt circuits as well as conventional 24 VAC circuits.

[edit] Digital thermostats with PID controller

More expensive models have a built-in PID controller, so that the thermostat knows ahead how the system will react to its commands. For instance, setting it up that temperature in the morning at 7am should be 21 degrees, makes sure that at that time the temperature will be 21 degrees (a conventional thermostate would just start working at that time). The PID controller decides at what time the system should be activated in order to reach the desired temperature at the desired time. It knows this by remembering the past behavior of the room, and the current temperature of the room.

It also makes sure that the temperature is very stable (for instance, by reducing overshoots at the end of the heating cycle) so that the comfortlevel is increased.

[edit] Commercial thermostats

In commercial applications, the thermostat may not contain any clock mechanism. Instead, another means may be used to select between the "hotter" and "colder" settings. For example, if the thermostat uses pneumatic controls, a change in the air pressure supplied to the thermostat may select between the "hotter" and "colder" settings, and this air pressure is determined by a central regulator. With electronic controls, a specific signal may indicate whether to operate at the "hotter" or "colder" setting.